In general, some boiling water reactors comprise an external recirculation system for forcibly feeding a coolant into a reactor core. A jet pump connected to the external recirculation system is provided in a reactor pressure vessel (RPV). A plurality of sets of jet pumps are provided in an annular downcomer portion between the reactor pressure vessel and a core shroud.
Each of the jet pumps is provided in the downcomer portion in the reactor pressure vessel, and comprises a riser pipe, a transition piece, an inlet mixer, and a diffuser. The transition piece is formed integrally on the riser pipe, to constitute a bifurcated portion on a head portion of the riser pipe. The coolant is divided into two streams in the transition piece, and respectively guided to an elbow of the inlet mixer that forms a turn-around flow path. The elbow is press-supported by a jet pump beam made of a spring member, and thereby prevented from moving upward.
In the boiling water reactor, inconel X-750 alloy that is precipitation-hardened Ni base alloy having a high elastic modulus and high-temperature strength has been conventionally used for the pump beam of the jet pump as disclosed in Japanese Patent Laid-Open No. 59-85834 (Patent Document 1). An example in which the inconel X-750 alloy is used for a reactor member is also disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 05-164886).
It is also disclosed in Japanese Patent Laid-Open Nos. 2004-91816 and 04-297537 (Patent Documents 3 and 4) that inconel 718 alloy that is normal heat-treated precipitation-hardened nickel base alloy is also used for a reactor member as a high-strength material having corrosion resistance and high-temperature resistance.
The inconel X-750 alloy as the precipitation-hardened nickel base alloy having a high elastic modulus and high-temperature strength is used for the jet pump beam of the jet pump disclosed in Patent Document 1, which is used under a high-temperature and high-pressure water environment in the boiling water reactor. Although the inconel X-750 alloy has a high elastic modulus and high-temperature strength as the material for constituting the jet pump beam, the inconel X-750 alloy has a problem that stress corrosion cracking (SCC) sensitivity is high under a high-temperature and high-pressure water environment, and stress corrosion cracking occurs under a high-temperature and high-pressure environment during operation of the nuclear reactor.
The inconel 718 alloy disclosed in Patent Document 3 as the nickel base alloy is used for the reactor member in a similar manner to the inconel X-750 alloy. It is disclosed that heat treatment on the inconel 718 alloy is performed by subjecting the material to solution heat treatment at 995° C. to 1040° C., keeping the material at 760° C. for 5 hours, cooling (furnace cooling) the material from 760° C. to 650° C. inside a furnace, keeping the material at 650° C. for 1 hour, and subjecting the material to age-hardening treatment for cooling.
ASTM (American Society for Testing and Materials) B637 also describes an example in which normal heat-treated inconel 718 alloy is subjected to solution heat treatment at 924° C. to 1010° C., kept at 718° C. for 8 hours, cooled inside a furnace, further kept at 620° C. for 8 hours, and then subjected to age-hardening treatment for air-cooling.
Although the normal heat-treated inconel 718 alloy subjected to the normal heat treatment is a material excellent in high-temperature strength as disclosed in ASTM or the like, the normal heat-treated inconel 718 alloy also has a problem that ductility is poor.